1. Field of the Invention
The present invention relates to an organic electroluminescent device that converts electric energy into light and emits the light and also a novel fused heterocyclic organic compound.
2. Description of the Related Art
Since organic electroluminescent devices (which may hereinafter be called “organic EL devices”) are capable of emitting light with a high luminance at low voltage, they have been actively researched and developed. Organic electroluminescent devices have a pair of electrodes and an organic layer therebetween and utilize, for light emission, energy of the exciton generated as a result of recombination of electrons injected from the cathode and holes injected from the anode in the organic layer.
The devices have recently had improved efficiency by using phosphorescent materials. There are disclosed inventions relating to phosphorescent devices using an iridium complex or a platinum complex as a phosphorescent material (refer to, for example, U.S. Pat. No. 6,303,238 and WO00/57676). However, devices that can satisfy both high efficiency and high durability have not yet been developed.
JP-A-2001-160488 discloses an invention relating to an organic electroluminescent device containing a compound having, as a dopant, a skeleton represented by the following formula (A):

JP-A-2001-357977 discloses an invention relating to an organic electroluminescent device containing a compound that contains a heterocycle skeleton having at a fused position thereof a nitrogen atom and is represented by the following formula (B):

In formula (B), X represents a carbon atom or a nitrogen atom, and Z1 and Z2 each independently represents a group of atoms capable of forming a nitrogen-containing heterocycle.
However, JP-A-2001-160488 and JP-A-2001-357977 do not disclose a compound having a plurality of pyrazoloimidazole skeletons.
For increasing practical usability, organic electroluminescent devices should be excellent in both external quantum efficiency and running durability. There is therefore a demand for further improvement of the devices.